(1) Field of the Invention
The present invention relates to a cage for a rolling bearing, in particular for a ball bearing, a rolling bearing with a cage, and the use of a cage in a high speed rolling bearing with a characteristic speed value n×dm>1,000,000 mm/min, whereby n corresponds to the inner ring speed and dm to the mean bearing diameter, whereby the mean bearing diameter dm is calculated from half the sum of the outside diameter and the bore diameter, in particular for industrial spindle-bearing applications and in the medical sector, especially in dental technology.
(2) Description of the Art
In the state of the art composite materials are described such as, e.g., fabric-reinforced phenolic resin that have a sufficient performance with respect to friction and wear to achieve very good lifespan values in high-speed ball bearing applications. The favorable properties of this material can be improved if in addition oil is incorporated in the cotton fabric, which in addition increases the lifespan in applications in which there is a lack of lubrication. One of the main problems in cages made of composite materials, for example, a fabric-reinforced phenolic resin with a fine weave cotton fabric, is that these materials are usually thermally stable only up to about 120° C. and steam pressure sterilization at, for example, 136° C. is therefore not possible. However, this presents a big problem in particular for applications in the medical sector.
Other plastics that are used as cage material in high-speed, medical technical applications are high-performance plastics such as polyetheretherketone (PEEK), polyetherketon (PEK), polyamide imide (PAI), and polyimide (PI), which in lubricated grade form can meet, by admixture of lubrication additives and fibers, the requirements in medical technology. However, these are only functional in as far as favorable tribological conditions can be guaranteed in the ball bearing.
However, very often a lack of lubrication prevails, especially in applications in the dental sector. The ball bearing that is used for this can often not be provided with grinding covers (stationary seal rings) since these attain high friction values during operation and the temperature in the ball bearing increases significantly as a result. However, in particular in the dental turbine handpiece, a temperature increase is not acceptable as a device temperature>38° C. is experienced as very unpleasant by dental technicians and dentists. These ball bearings can therefore not be sealed but only covered. such a cover disk, however, has the disadvantage that a definite gap exists between the cover disk and the inner ring through which the lubricant can escape to the outside (see FIG. 1). In the medical sector, in particular in dental turbine bearings, the effect of the lubricant leakage is amplified by the fact that the supply air of the turbine cannot only escape to the outside through the ventilation opening but also through the turbine bearing. This leads to a transport of the lubricant (lubricant grease or oil) to the outside and thereby to the above-described situation with a lack of lubrication.
For example, in DE 198 48 051 A1 and in U.S. Pat. No. 6,113,278 a high-performance thermoplastic (PI and PAI, respectively) produced by means of a sintering technique is described that is equipped with a defined free pore volume. A suitable lubricating oil is incorporated by vacuum impregnation in the free pore space. This lubricating oil must, precisely in the situation with a lack of lubrication, exit from the pore space due to centrifugal forces (during the dynamic operation of the bearing) and thus maintain the tribosystem in the ball bearing. However, the described mechanism has not proven itself in practice, because, on the one hand, the cage becomes very fragile due to the porosity and, on the other hand, the “oil storage effect” does not last long, because the lubricant is very quickly washed out of the pore space by regularly carried out pressure steam sterilization cycles.
Compounds of polyamide and perfluoroalkyl substance(s) and mixtures of these compounds with additional polymer substances are described in DE 198 23 609 A1, whereby modified perfluoroalkyl substance(s) is(are) homogenized with polyamide compound(s) in the molten state by means of a reactive conversion. The use of this material thereby can be as a pure substance or as an additive/ingredient in sliding bearings, in sliding films, in sliding foils, in sliding coatings, in oil-repellent and/or hydrophobics, or component or full body materials that are provided with it, in moldings, in textile threads and/or fleece and/or other textile fabrics, in multi-layer (foil) materials, in membranes, as a coating additive or a coating substance.
Plastic sliding bearings, as mentioned in DE 198 23 609 A1, can be used when a relative motion must be transferred without lubricants (lubricating oil or fat) between the two bearing partners, e.g., the shell and the shaft. The specific surface pressures are not particularly high compared to the plastic sliding bearing after the shaft supports itself with its entire lateral surface in the bearing shell. Even in high performance sliding bearings maximum peripheral speeds of only 1.5 m/s are used. Converted to the size of an example of an, in dental technology used, bearing with a bore diameter of 3.175 mm and an external diameter of 6.35 mm, this results only in a characteristic speed value of about 30,000 mm/min. However, high-performance rolling bearings, such as dental turbine bearings, rotate at a speed of up to 500,000 min−1 which corresponds to a characteristic speed value of about 2,400,000 mm/min. This example illustrates that the performance data of a high-performance rolling bearing, such as a dental turbine bearing, are not in any way comparable to those of a high-performance sliding bearing.